Re: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in shm_mmap()

[Davidlohr Bueso]

On Wed, 11 Nov 2015, Kirill A. Shutemov wrote:

> remap_file_pages(2) emulation can reach file which represents removed
> IPC ID as long as a memory segment is mapped. It breaks expectations
> of IPC subsystem.
>
> Test case (rewritten to be more human readable, originally autogenerated
> by syzkaller[1]):
>
> 	#define _GNU_SOURCE
> 	#include <stdlib.h>
> 	#include <sys/ipc.h>
> 	#include <sys/mman.h>
> 	#include <sys/shm.h>
>
> 	#define PAGE_SIZE 4096
>
> 	int main()
> 	{
> 		int id;
> 		void *p;
>
> 		id = shmget(IPC_PRIVATE, 3 * PAGE_SIZE, 0);
> 		p = shmat(id, NULL, 0);
> 		shmctl(id, IPC_RMID, NULL);
> 		remap_file_pages(p, 3 * PAGE_SIZE, 0, 7, 0);
>
> 	        return 0;
> 	}
>
> The patch changes shm_mmap() and code around shm_lock() to propagate
> locking error back to caller of shm_mmap().
>
> [1] http://github.com/google/syzkaller

So this is a very similar approach that I posted back when this discussion
arose: https://lkml.org/lkml/2015/10/12/959 -- There are a few differences
for which I prefer mine :)

o My shm_check_vma_validity() also deals with IPC_RMID as we do the
ipc_valid_object() check.

o We have a new WARN where necessary, instead of having one now is shm_open.

o My no-ops explicitly pair.

[...]

> 	ret = sfd->file->f_op->mmap(sfd->file, vma);
> -	if (ret != 0)
> +	if (ret) {
> +		shm_close(vma);
> 		return ret;
> +	}

Hmm what's this shm_close() about?

Thanks,
Davidlohr
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